With the recent demand to reduce the size and increase the degree of integration of semiconductor devices through the development of semiconductor manufacturing technology, techniques for forming ultrafine patterns having line widths corresponding to tens of nm or less are required. Advances in techniques for forming ultrafine patterns have been made by the use of light sources having smaller wavelengths and through the development of processing technology and photoresists suitable for such light sources.
A photoresist is used for photolithography for forming a variety of patterns. The term “photoresist” refers to a photosensitive resin, the solubility of which varies in a developing solution due to the action of light, to thus obtain an image corresponding to the exposure pattern.
The formation of a photoresist pattern includes negative tone development (NTD) using a negative tone developing solution and positive tone development (PTD) using a positive tone developing solution.
The process of forming a pattern through NTD includes selectively dissolving and removing an unexposed region using a negative tone developing solution, and the process of forming a pattern through PTD includes selectively dissolving and removing an exposed region using a positive tone developing solution.
When compared to pattern formation using PTD, pattern formation using NTD enables the formation of a reverse-phase pattern in a contact-hole pattern or a trench pattern, which is considered to be difficult to form due to insufficient exposure, thereby making it easy to consistently form a pattern. Furthermore, an organic solvent is used as the developing solution for removing the unexposed portion, thereby more effectively forming a photoresist pattern.
Meanwhile, a typical photolithography process using a photoresist composition includes coating a wafer with a photoresist, performing soft baking for heating the coated photoresist to evaporate the solvent, forming an image by means of a light source passed through a photomask, forming a pattern through a difference in solubility between an exposed portion and an unexposed portion using a developing solution, and completing a circuit through etching.
The photoresist composition is composed of a photosensitive agent (a photoacid generator) for generating an acid through excimer laser irradiation, a basic resin and other additives. The basic resin, configured such that a phenol structure contains a hydroxyl group, typically includes a polystyrene polymer, and any photosensitive agent may be used, so long as it is able to generate an acid (H+) at a specific wavelength, and main examples thereof may include sulfonium-, sulfonyl diazo-, benzo sulfonyl-, iodine-, chlorine-, and carboxylic acid-based organic and inorganic acids.
However, a negative photoresist obtained using the above composition is problematic in that the photosensitive agent therein is unable to generate a sufficient amount of acid (H+), making it impossible to form a desired shape, and also in that a profile deteriorates upon the formation of a finer pattern.
Also, a light source mainly used for the above process has a wavelength range of 365 nm to 193 nm, examples of which include an I-ray, a KrF excimer laser, and an ArF excimer laser. As is known in the art, the shorter the wavelength, the finer the pattern.
In particular, thorough research into a KrF laser (243 nm) photoresist for photo microprocessing is ongoing despite the development of an ArF laser (193 nm) system. This is because the development of next-generation ArF photoresists is still unsatisfactory and also because the use of a KrF photoresist may greatly reduce the cost for mass production of semiconductors. Accordingly, the performance of a KrF photoresist has to be improved in response to such technical development, and, for example, the thickness of the photoresist is required to decrease with an increase in the degree of integration. Hence, the development of a photoresist in which dry etching resistance is further increased is urgently required. In addition thereto, there are requirements for high resolution, a wide DOF (Depth Of Focus) margin, defect-free thin film formation, adhesion to a substrate, high contrast, fast sensitivity and chemical stability.
Conventional techniques pertaining to the negative photoresist for a KrF laser include Korean Patent No. 10-0266276 ┌Negative photoresist composition┘, Korean Patent Application Publication No. 10-2015-0067236 ┌Negative photosensitive resin composition┘, Korean Patent Application Publication No. 10-2015-0047433 ┌Salt and photoresist composition including same┘, and Korean Patent Application Publication No. 10-2015-0026996 ┌Compound, resin, photoresist composition, and method of forming photoresist pattern┘.
As disclosed in the conventional patents, a photoresist for KrF is mainly composed of polyhydroxystyrene and polystyrene polymers having good transmittance at a wavelength of 248 nm in order to increase resolution and sensitivity.
However, the photoresist composed of polyhydroxystyrene and polystyrene polymers is problematic because the formed pattern is comparatively loose and thus a fine pattern having high aspect ratio may easily collapse, making it difficult to form a pattern having high resolution and a high aspect ratio.